I don't agree. To me our attack doesn't look thin at all. It's unproven, but that's a very different matter (one which will affect ManU as well when their comings and goings have been completed), and IMHO it has at least as good a chance of outshining theirs as vice versa. Not sure I understand what you're getting at with regard to Mata - more productive than Suarez by what criterion?
Mata's Chelsea stats:
2011-12 - 12 goals, 20 assists
2012-13 - 20 goals, 28 assists
That's a goal involvement rate of 40 per season. Higher than Suarez's at Liverpool.
As for our attack not being thin, I find that baffling. Assuming Borini leaves, we have two strikers at the club - Sturridge and Lambert, one of whom is injury prone, and the other 32 and hardly top class.
United have 4 - van Persie, Rooney, Hernandez and Welbeck - 3 of whom are proven goalscorers at the top level.
In the AM/winger category, we have good depth of course - Lallana, Sterling, Coutinho and Markovic. That's a very solid bunch of options, and a few of those will hopefully be world class in 2 years.
But United have equally good depth, although the quality is a bit varying (from crap to world class) - Mata, Kagawa, Januzaj, Young and Valencia is excellent strength in depth. You may discount Young and Valencia, but Mata atm is far ahead of Lallana and Kagawa and Januzaj are comparable talentwise to Coutinho and Sterling (though I'd prefer our pair).
On the whole, their attack is better on paper. There isn't a question about it in my mind. The question is whether the Moyes effect will continue to linger.
Of course this might all change come the end of the transfer window.